While cause of MS is still not known, extensive scientific research indicates that a combination of several factors may be involved. The major scientific theories about the causes of MS include the following:

IMMUNOLOGIC: It is now generally accepted that MS involves an autoimmune process—an abnormal immune response directed against the central nervous system (CNS); the brain, spinal cord and optic nerves. The exact antigen, or target that the immune cells are sensitized to attack, remains unknown. In recent years, however, researchers have been able to identify which immune cells are mounting the attack, some of the factors that cause them to attack, and some of the sites, or receptors, on the attacking cells that appear to be attracted to the myelin to begin the destructive process. The destruction of myelin—the fatty sheath that surrounds and insulates the nerve fibers—as well as damage to the nerve fibers themselves, cause the nerve impulses to be slowed or halted and produce the symptoms of MS. Researchers are looking for highly specific immune-modulating therapies to stop this abnormal immune response without harming normal immune functions.

ENVIRONMENTAL: Migration patterns and epidemiologic studies—those that take into account variations in geography, socioeconomics, genetics, and other factors—have shown that people who are born in an area of the world with a high risk of MS and then move to an area with a lower risk before the age of 15, acquire the risk of their new home. Such data suggest that exposure to some environmental agent that occurs before puberty may predispose a person to develop MS later on.

MS is known to occur more frequently in areas that are farther from the equator. Some scientists think the reason may have something to do with vitamin D, which the human body produces naturally when the skin is exposed to sunlight. People who live closer to the equator are exposed to greater amounts of sunlight year-round. As a result, they tend to have higher levels of naturally-produced vitamin D, which is thought to have a beneficial impact on immune function and may help protect against autoimmune diseases like MS. The possible relationship between MS and sunlight exposure is currently being looked at in a Society-funded epidemiological study in Australia.

INFECTIOUS AGENT: Since initial exposure to numerous viruses, bacteria and other microbes occurs during childhood, and since viruses are well recognized as causes of demyelination and inflammation, it is possible that a virus or other infectious agent is the triggering factor in MS. More than a dozen viruses and bacteria, including measles, canine distemper, human herpes virus-6, Epstein-Barr, and Chlamydia pneumonia have been or are being investigated to determine if they are involved in the development of MS, but as yet none has been definitively proven to trigger MS.

GENETIC: While MS is not hereditary in a strict sense, having a first-degree relative such as a parent or sibling with MS increases an individual's risk of developing the disease several-fold above the risk for the general population. There are studies that show there is a higher prevalence of certain genes in populations with higher rates of MS. Common genetic factors have also been found in some families where there is more than one person with MS. Some researchers theorize that MS develops because a person is born with a genetic predisposition to react to some environmental agent that, upon exposure, triggers an autoimmune response. Sophisticated new techniques for identifying genes may help answer questions about the role of genes in the development of MS.